Programmable current sources, often implemented using current mirrors, are conventionally used to control current to a number of different types of loads. For example, wideband low phase noise VCO's are required in many modern communication systems. These VCO's are often implemented as LC tuned VCO's because they provide relatively good phase noise performance. Since phase noise degrades with increased tuning sensitivity (Kvco), wideband designs are often done with a switchable bank of capacitors in parallel with the varactor. The capacitor bank splits the frequency range of the VCO into small bands. This allows the VCO to cover a large range of frequency with a small Kvco.
For optimum phase noise for a given bias current it is desirable to operate the VCO in the current-limited regime and not in the voltage-limited regime. With a wideband VCO the tank amplitude varies significantly with frequency and therefore it is desirable to have a method to control the oscillation amplitude. The amplitude of the oscillation in the current-limited regime is controlled by the bias current in the oscillator. For a fixed bias current the amplitude of oscillation in the current-limited regime increases with tank impedance which generally increases with frequency. The bias current can be digitally controlled, by replacing either the bias source or current mirror with a current source DAC. Either method allows a different bias to be selected depending on the frequency band of the VCO.
In the case where the bias source is replaced with a DAC, the vdsat of the mirror transistors will decrease with the bias current of the VCO. In the case where the mirror is replaced with a DAC, the vdsat of the transistors will remain constant with bias current.
Noise in the bias current of a VCO can be a significant source of phase noise in the oscillator. The conversion of low-frequency bias noise to phase noise in the oscillator is dependent on the transition frequency (ft) of the core transistors. Since a fixed width transistor has a lower ft with a lower bias current, the conversion of bias noise to phase noise is the worst when the bias current is lowest.